Have Nectar Sugar Concentrations Evolved to Match Pollinator Preferences

The cost of producing nectar can be substantial, and at least in environments of low water stress, this cost can be directly related to sugar content [114]. Thus, if total sugar mass is held constant, it costs a plant the same amount to offer a pollinator 35% sugar as it does 65% sugar [115]. If nectar-feeding insects seek to maximize their rate of energy intake during feeding, then they should prefer to visit plants that provide nectars matching their optimal sugar concentration. Consequently, flowers specializing on a particular pollinator may be expected to evolve sugar concentrations that match pollinator preferences.

Euglossine bees are derived suction feeders with an optimal nectar sugar concentration that falls between 30 and 40% sucrose [31]. We compiled data on the nectar sugar concentrations recorded from flowers in 28 species in 9 families that euglossine bees are known to visit and categorized these flowers as euglossine specialists or generalists (Table 9.5). Overall, we found a close match between optimal nectar sugar concentrations and the concentrations found in specialist flowers. More significantly, however, we observed lower variance in sugar concentrations in specialist as compared to generalist flowers, but we caution that verifying this trend requires additional data and phylogenetic controls. In comparison with sympatric bees that lap nectars, eugloss-ine bees also tend to forage from flowers with more dilute rewards (Table 9.6). Other analyses of floral nectars have supported partitioning of pollinator guilds on the basis of sugar concentration [89,98,116], but as is evident from Table 9.6, feeding biomechanics is clearly only one factor influencing these trends. Opposing physiological pressures to minimize water loads in flight [117] or obtain dietary water [89,118,119] may also influence choice behavior by nectarivores and the evolution of nectar sugar concentrations in flowers. Additionally, floral generalization, recent pollinator shifts, and phylogenetic inertia may contribute to the mismatch between sugar concentration and feeding mechanics in some taxa.

One method for assessing how the biomechanics of nectar ingestion has influenced nectar constituents of flowers is to evaluate choice behavior of nectarivorous

TABLE 9.5

Mean Nectar Sugar Concentrations (% w/w) for Some Flowers Visited by Euglossine Bees

TABLE 9.5

Mean Nectar Sugar Concentrations (% w/w) for Some Flowers Visited by Euglossine Bees

Family

Genus (Species N)

Specialist

Sucrose (%)

Ref.

Apocynaceae

Stemmadenia (1)

N

34

142

Apocynaceae

Thevetia (1)

N

32

142

Bignoniaceae

Jacaranda (1)

N

15

B.J. Borrell,

unpublished

Bignoniaceae

Tabebuia (3)

N

39

142

Convolvulaceae Ipomoea (1)

N

31

143

Gesneriaceae

Drymonia (2)

N

34

12, 144

Gesneriaceae

Sinningea (2)

N

26

145

Mimosaceae

Inga (2)

N

26

105, 146

Passifloriaceae

Passiflora (2)

N

40

107

Costaceae

Costus (4)

Y

36

12, 106

Costaceae

Dimerocostus (1)

Y

35

B.J. Borrell,

unpublished

Gesneriaceae

Sinningea (2)

Y

34

145

Lecythidaceae

Coratari (1)

Y

39

147

Lecythidaceae

Eschweilera (2)

Y

36

147

Marantaceae

Calathea (3)

Y

38

148-150

Mean (± 95%

C.I.) for generalist flowers

31 ± 5.7

Mean (± 95%

C.I.) for euglossine specialists

36 ± 1.7

Note: In general, nectars were extracted from new flowers during times of pollinator visitation, and the equivalent sucrose concentration was measured using a handheld refractometer. Designation of flowers as euglossine specialists was based on visitation frequency data reported by the authors, not taking into account pollinator efficiency.

TABLE 9.6

Mean Nectar Sugar Concentrations (% w/w) of Flowers Visited by Different Animal Taxa in a Variety of Habitats

TABLE 9.6

Mean Nectar Sugar Concentrations (% w/w) of Flowers Visited by Different Animal Taxa in a Variety of Habitats

Common Name

Feeding Mode

Sucrose (%)

Habitat Type

References

Bumblebee

Lapping

44

Temperate meadow

151

Centridine bee

Lapping

48

Tropical forest

102

Stingless bee

Lapping

44

Tropical forest

102

Hummingbird

Lapping

22

Tropical wet forest

89

Leaf-nosed bat

Lapping

14

Tropical wet forest

152

Orchid bee

Suction

36

Tropical wet forest

See Table 9.5

Long-proboscid fly

Suction

26

Mediterranean shrub

15

Hawkmoth

Suction

22

Tropical dry forest

11

Butterfly

Suction

25

Temperate

26

insects in laboratory studies. Numerous investigations have measured visitation rates of nectarivores to effectively infinite volume sucrose solutions and concluded that these animals prefer the most concentrated solutions offered them [96,120-122]. One problem with this approach is that it confounds nectar sugar concentration with total meal energy [75,123]. The more relevant question is how much water should a plant add to a fixed quantity of sugar in order to maximize attractiveness to pollinators [115]. Furthermore, behavioral studies should use realistic nectar volumes and monitor transport costs to and from nectar sources so that the data may be analyzed for a variety of timescales [26,77]. Roberts' exemplary study of hummingbird foraging [123] analyzed concentration preferences at different timescales but employed an equal volume rather than an equal sugar design. Hainsworth and Hamill [75] conducted the only published sugar choice experiment we know of by offering the butterfly Vanessa cardui a choice between feeding from a 70% solution for 30 sec or a 35% solution for 20 sec. In spite of the decline in energy intake rate, these authors found that butterflies still preferred the more concentrated solution. One caveat with interpreting these results is that butterflies were not freely foraging but were captured and hand fed upon landing at color-coded feeding sites. The euglossine bee Euglossa imperialis does not discriminate between 35% (feeding time [FT] = 9 sec) and 55% (FT = 15 sec) solutions offered in an equal sugar design; Euglossa imperialis does, however, show a slight but significant preference for 35% (FT = 9 sec) sucrose over 60% (FT = 30 sec; B.J. Borrell, unpublished). Neither B.J. Borrell (unpublished) nor Hainsworth and Hamill [75] monitored transport costs, which when taken into account, predict preferences for more concentrated nectars than consideration of feeding costs alone [26].

An alternative route of investigation has been to augment the viscosity of pure sucrose solutions using small quantities of polymers such as tylose or methyl cellulose [97,124,125]. Hummingbirds do not distinguish between 20% sucrose solution and a 20% sucrose solution with the viscosity increased to that of a 40% solution [125]. However, the bee Euglossa imperialis shows a strong preference for low viscosity nectars in choice experiments (B.J. Borrell, unpublished).

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